I've been a technician for a few years and I've always notice A system hydraulic on the NG's (700 and 800) are almost always overserviced during layover svc chks. I remember back at Delta a few years ago we had a read and sign about overservicing A side but I forgot the reason behind it. The hyd qty gauge would read 1.06 but actual qty would be much higher.
I know techs are not over svcing A side. But some are at fault for svcing B side instead of transfering some fluid back from A. My current employer has the flight crew taxi in with #1 engine to facilitate a quicker turn around. They do not set parking brakes when parked as wheels are chocked.
Anyone have any insight on this? I know this is also a common issue with the 757 to but I just can't remember the reason behind it. I do remember T/S a 757 hyd transfer issue. It's been a few years but I remember the fim calling out a procdure to check for alernate brakes shuttle valve leakage (correct me if i'm wrong, been awhile). None was noted. I believe the 737ng fim calls for a similar senario but I have a hard time believing a whole fleet would have this issue.

The views and opinions written here are my own and do not reflect those of my employer.

Quoting FADECFAULT (Thread starter): I know this is also a common issue with the 757 to but I just can't remember the reason behind it. I do remember T/S a 757 hyd transfer issue. It's been a few years but I remember the fim calling out a procdure to check for alernate brakes shuttle valve leakage (correct me if i'm wrong, been awhile).

It's been awhile since I have seen a 73NG here in Boston, The 757 and 767, you can move fuild back and forth just by using the brakes, Brakes on with the system your moving from, bleed off pressure and Brakes off to the system you want to move it too,(pumps on), It takes about 15 minutes to move about .10 of fluid on the EICAS

One reason for the A system to be overfilled is that it is a smaller system. The reservoir on the A system is smaller than on the B system. If my memory serves, one tank is 6.8 gallons and the other is around 8, but that's coming out of my not so good memory.

The only way to transfer fluid between is through the use of the thrust reversers in the way that they are deployed. All other systems are completely independant. There are shuttle valves in place in the brakes which prevent fluid transfer between the two hydraulic systems.

Taxiing on one engine should have zero effect on hydraulic fluid quantity.

The maximum read out is 106%, so yes it can be higher than that. Also, you have to make sure that the reservoirs are presurrized because otherwise the reading in the flight deck is not correct.

Quoting FADECFAULT (Thread starter):I do remember T/S a 757 hyd transfer issue. It's been a few years but I remember the fim calling out a procdure to check for alernate brakes shuttle valve leakage (correct me if i'm wrong, been awhile).

The brakes will not transfer fluid between the A and B system on the 737.

If you have never designed an airplane part before, let the real designers do the work!

Quoting RoseFlyer (Reply 2):The brakes will not transfer fluid between the A and B system on the 737

I don't beleive that is correct. The Alternate brakes shuttle valve links the two to a common point. Just like the 757 and 767 you can transfer fluid by engaging and disengaging the parking brake. I do it all the time. Except with the 737 I don't turn on B pump and release the brakes.

The views and opinions written here are my own and do not reflect those of my employer.

There are a number of methods for transferring fluid from one system to another. The brakes are one.
In the instance you bring up it sounds like your aircrews are not adhereing strictly to the Boeing published AOM and there may be an inadvertant transfer of fluid after landing and one engine shutdown or any of a number of other times during taxi to the gate.
A number of operators have experienced this phenomenon just after their transition from classics or jurassics to NG's.

It should go without saying that if a hydraulic system is low on quantity then you should top up that system with fresh fluid (and find out why it was low!) to avoid cross contamination. However if you really want to move fluid from one system to another here is how to do it.

A to B (1% transfer per cycle)

Chock the aircraft & ensure area around stabiliser is clear.
Switch both EMDP's OFF.
Release parking brakes and deplete accumulator to below 1800psi by pumping toe brakes.
Switch Sys A EMDP ON and apply parking brakes.
Switch Sys A EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel)
Switch Sys B EMDP ON and release parking brakes. (Sends the fluid back to system B because the shuttle/priority valves send the fluid back to the normal brake system.)
A to B - An alternative method

Chock the aircraft & ensure area around stabiliser is clear.
Switch both EMDP's ON.
Switch Sys B EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel)
Switch Sys A EMDP ON and apply parking brakes. (Uses fluid from system A)
Switch Sys B EMDP ON and release parking brakes. (Sends the fluid back to system B because the shuttle/priority valves send the fluid back to the normal brake system.)
B to A (4% transfer per cycle)

Boeing aircraft only have one set of brake pistons on the brake units. So any brake system will pressurise the same cylinders. When the brakes are released, the return fluid will go bake to the normal reservoir.
You can also transfer fluid on B777 and B747 with the brakes.
The way to stop this happening is to train your flight crew to turn on the hyd systems in the correct order.
On a B757 you must turn on the R system first. Our crews don't. They start with L then C then R. This ensures that they are always transferring fluid from L tank to R tank, and we move it back again.
Airbus are different as they have two sets of brake cyclinders on each brake unit. There is one set for Green system, and one for Yellow system, so you don't get fluid transfer through the brakes.
Also on an Airbus, the park brake pressure does not decay overnight, unlike a Boeing where leakage past the park brake valve will drop the parking pressure to zero after a nightstop.

Quoting Avioniker (Reply 4):Chock the aircraft & ensure area around stabiliser is clear.
Switch both EMDP's OFF.
Release parking brakes and deplete accumulator to below 1800psi by pumping toe brakes.
Switch Sys A EMDP ON and apply parking brakes.
Switch Sys A EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel)
Switch Sys B EMDP ON and release parking brakes. (Sends the fluid back to system B because the shuttle/priority valves send the fluid back to the normal brake system.)
A to B - An alternative method

Chock the aircraft & ensure area around stabiliser is clear.
Switch both EMDP's ON.
Switch Sys B EMDP OFF and depressurise through control column. (Use stabiliser rather than ailerons to prevent damage to equipment or personnel)
Switch Sys A EMDP ON and apply parking brakes. (Uses fluid from system A)
Switch Sys B EMDP ON and release parking brakes. (Sends the fluid back to system B because the shuttle/priority valves send the fluid back to the normal brake system.)
B to A (4% transfer per cycle)

Thanks for the information. I never thought about using the parking brake to transfer fluid between the systems. You always learn more on A.net from people finding and using workarounds to solve problems.

If you have never designed an airplane part before, let the real designers do the work!

Quoting FADECFAULT (Thread starter): I've always notice A system hydraulic on the NG's (700 and 800) are almost always overserviced during layover svc chks.

Keep the Flaps in retracted position prior to servicing.

Quoting FADECFAULT (Thread starter):I know this is also a common issue with the 757 to but I just can't remember the reason behind it. I

RH system operates Normal brakes & LH is Alternate brakes.Switching the Parking brakes with one system & releasing with another system pressurised can cause the transfer.
On B752 Turn on the RH pumps first & off last.

We had a few guys use the elec pumps going off and on with cycling the brakes but it burned up a relay (if I recall correctly) and caused a delay in the morning awaiting replacement parts.

What I generally do to transfer A to B is as follows....

With A elec pump on constantly throughout the procedure, I would push and hold the brakes, pull the speed brake handle to deploy, when they are deployed, I would release pressure on the brakes. Then do the same while stowing the speed brake handle. Rapidly doing up and down with the speed brake handle with the brakes being held down while they cycle up/ down. Seems to transfer 1% per cycle.

Now from B sys to A, I would do as another poster mentioned, I would use the Standby Hyd and deploy the #1 reverser and stow it with the normal A elec pump. Would transfer around 3% per cycle if I recall.

The reason you don't want to over service a NG is because unlike a Classic 737, the hydraulics are not vented overboard for an overfill situation. If the acft hasn't had the modification, a check-vlave installed (not sure of the mod name or number) being overserviced, it would push fluid back into the pneumatic system via the head-pressure air line causing smoke/fumes in the cabin and all the pneumatic ducts would have to be opened and cleaned before the acft could re-enter revenue service again... In other words, long down time in the hanger!

We should have all our 300+ NG's mod'ed by now. New builds were coming delivered with the mod in place, so we were having to do older NG's. This is as per one of our big wheels at HDQ, I'm just a lowly line mechanic at an out-station... So not in on all the goings on at the major bases..